Abstract [en]

A critical step in ecological speciation is the development of reproductive isolation. The processes leading up to reproductive isolation are difficult to study since they often occur over very long time periods. Populations of European whitefish (Coregonus lavaretus) has gone through reticulate divergent evolution in many Scandinavian lakes during the last 10 000 years. Some populations even exhibit morphological and genetic divergence that involves adaptations to different niches during the first 100-200 years after introduction. This observed rapid diversification into different ecological niches makes whitefish a useful model species for studying ecological speciation and early population divergence. By assessing divergence for three traits in recently introduced whitefish populations, this study aims to elucidate the processes that lead to adaptive phenotypic divergence and reproductive isolation. Whitefish populations of known age (71-170 years) were sampled on their spawning grounds and characterized with respect to, 1) length of spawning season, 2) temporal segregation based on body size, and 3) temporal segregation based on the number of gill rakers. I found that the length of the spawning season and body size differences between early and late spawners increased with population age. No such trend was observed for differences in gill rakers between early and late spawners, but significant divergence within some of the older populations was detected. I conclude that these young whitefish populations have taken the first steps toward reproductive isolation between ecotypes that differs in body size (a highly plastic trait) and gill raker numbers (a trait under strong genetic control).